The present invention relates to an information recording disc and a method of producing the same and, particularly, to an information recording disc adapted to record information by irradiating a major surface of the disc with a scannng beam such as laser beam of electron beam and a method of producing the same.
As a recording system of video or audio information on a recording medium in the shape of a disc, there has been proposed an optical system which has been utilized effectively.
The optical recording system utilizes a glass disc substrate on which a photoresist layer having thickness of 1000 to 1500 A is provided by painting. A laser scanning beam is collimated and directed to the photoresist layer so that a fine spot is formed on the layer. The laser beam is modulated with video or audio information to be recorded so that the photoresist layer is irradiated in the so-called bit-by-bit manner. Thereafter the disc is developed to obtain pits whose length and repetitive rate represent the information. The depth of the pit corresponds to the thickness of the photoresist layer.
In this method, the information on the disc is transferred to a suitable plate of such as plastics from which replicas are produced. Therefore, the thickness of the photoresist on the disc surface must be uniform. Otherwise the S/N ratio of reproduced signal may be highly degraded. However, it is very difficult to provide a uniform photoresist layer by painting over the whole area (diameter being about 300 mm) of the disc.
One conventional method which eliminates the difficulty of providing photoresist layer of uniform thickness is to epitaxially grow a preferentially etachable layer such as silicon oxide layer on one of the major surfaces of the disc and then a chrome layer which is capable of being evaporated by an irradiation of laser beam is formd on the oxide film so that the chrome layer is selectively evaporated according to the laser beam modulated with the information. The oxide layer is etched using the chrome layer thus evaporated selectively as an etching mask until the etching reaches at the substrate and then the mask is removed, resulting in the silicone oxide layer containing the pits corresponding to the information and each having depth corresponding to the thickness of the oxide layer.
In this method, the mentioned difficulty of providing a uniform photoresist layer is overcome because of the nature of the epitaxial growth. However, in either of the methods, there is a tendency of the pit size to increase in comparison with a normal size to be produced and therefore, an exact recording of information may become impossible.
Further, replicas of the disc thus obtained are produced by transferring the pits to a suitable transfer disc of such as plastic and by transferring the pattern on the transfer disc to other discs. Since walls of the pits formed in the silicone oxide layer are substantially orthogonal to the disc surface, the plastic transfer disc for replica which is formed by molding is not easily separated from the mother disc.